This invention relates to the manufacture of oriented plastics tubes, and in particular to the manufacture of tubes having a high degree of orientation in the circumferential direction.
International Patent Application No. WO 90/02644 describes a process for the manufacture of thermoplastics made, for example, of unplasticised polyvinyl chloride (uPVC) which have a degree of orientation in the circumferential direction that improves properties such as resistance to hoop stresses, and renders the tubes particularly suitable for transmission of water. The process described in that patent application comprises:
(i) extruding a tube of plastics material; PA0 (ii) temperature conditioning the extruded tube to bring it to a consistent temperature profile about the tube suitable for expansion of the tube to cause molecular orientation of the polymer, preferably to a temperature of 85-115.degree. C. for uPVC, more preferably 90-100.degree. C. PA0 (iii) diametrically expanding the tube by application of an internal pressure to the tube that is limited at its downstream end by a plug that is inflatable or otherwise expandable to maintain pressure within the expansion zone; and PA0 (iv) cooling the expanded tube to set the tube in its diametrically expanded configuration. PA0 (a) The absolute velocity of the unexpanded tube upstream of the expansion zone and the expanded tube downstream may be measured and compared. Preferably the upstream measurement is taken at a point substantially immediately before the expansion zone. This measurement will detect increases in expansion of the tube at the expansion zone as this will be accompanied by an increase in the velocity of material entering the expansion zone as the outward bulging will start to draw material from the upstream side of the upstream plug. In practice, the downstream velocity can be considered to be known with sufficient accuracy from the haul-off rate as no further stretch occurs in the cooled section of tube downstream of the expansion zone. Thus, a reasonable approximation of the relative velocity can be determined by detecting the upstream velocity alone, and variations in the upstream velocity alone may be used as the control parameter as the haul-off rate is dictated by the downstream haul-off tractor and will not generally fluctuate other than due to deliberate change of the haul-off rate. PA0 (b) The diameter of the partly expanded tube may be measured at a position within the expansion zone, preferably at a position less than about three times the pre-expansion tube diameter downstream of the upstream plug. This diameter reflects the precise configuration of the generally frusto-conical expansion region. For example, if the velocity of tube leaving the expansion zone reduces relative to the delivery to the expansion zone, the frusto-conical expansion region of the tube will bulge outwardly to some extent thereby increasing the measured diameter, while if velocity leaving the expansion zone increases relative to the delivery to the expansion zone, the frusto-conical expansion region will become more axially elongated and the measured diameter will decrease. PA0 (c) The force exerted by the expanded tube on the sizing device in the axial direction may be measured. In a first mode of operation the tube expands to fit the sizing device and exerts an axial force on the sizing device in the downstream direction due to its shearing against the internal surface of the sizing device. The magnitude of the axial force will depend on the axial length over which the expanded tube contacts the sizing device and/or on the radial pressure which it applies to the device so that appropriate feedback control can maintain the point at which the tube first contacts the sizing device at the set point. In a second mode of operation, the tube is slightly overexpanded before being drawn down in diameter through the sizing sleeve. The axial force on the sizing sleeve will reflect both the deformation of the overexpanded tube to enter the sizing device and shearing against the inner surface. In either operative mode, the measured force is related to the shape of the expansion zone, and thus to the relative velocities as described above. PA0 (d) The diameter of the unexpanded tube just prior to the expansion zone, for example less than about 10 times the pre-expansion tube diameter upstream of the upstream plug, may be measured. This reading reflects the long term average in velocity entering the expansion zone, as too high an entry velocity over a period of time will cause material to be drawn from the pre-expansion zone, resulting in thinning of the tube material and reduction of diameter upstream of the expansion zone.
At the downstream end of the expansion zone, the expanded tube passes through a circular passage in a sizing device which limits diametrical expansion of the tube to that which allows the expanded tube to slide through the sizing device. After the tube has passed through the expansion zone and while it is being cooled, the tube may contract in diameter by a small amount, e.g. a few millimeters or less (so-called "snap-back") as the internal pressure is reduced, to give the tube its final, expanded, diameter.
In the case of tubes that are intended to be used as water pipes, and which must therefore be joined end-to-end, it is necessary for the diameter of the expanded tube, and especially its external diameter, to be controlled in order that the tube will be able to fit accurately and tightly within a coupling device or within an additionally expanded end section of an adjacent tube so as to prevent leakage. We have found, however, that control of the expansion process using conventional feedback control, i.e. by measuring the final expanded diameter and wall thickness of the tube, does not control the properties of the resultant tube sufficiently accurately, nor allow stable operation of the process itself.